1. Field of the Invention
The invention relates to a process for production of polycrystalline silicon.
2. Description of the Related Art
Polycrystalline silicon (polysilicon for short) serves as a starting material for production of monocrystalline silicon for semiconductors by the Czochralski (CZ) or zone melting (FZ) process, and for production of mono- or polycrystalline silicon by various pulling and casting processes for production of solar cells for photovoltaics.
Polycrystalline silicon is generally produced by means of the Siemens process. In this process, in a bell jar-shaped reactor (“Siemens reactor”), support bodies, typically thin filament rods of silicon, are heated by direct passage of current and a reaction gas comprising hydrogen and one or more silicon-containing components is introduced.
Typically, the silicon-containing component used is trichlorosilane (SiHCl3, TCS) or a mixture of trichlorosilane with dichlorosilane (SiH2Cl2, DCS) and/or with tetrachlorosilane (SiCl4, STC). Less commonly, but on the industrial scale too, silane (SiH4) is used.
The filament rods are inserted perpendicularly into electrodes at the reactor base, through which they are connected to the power supply.
High-purity polysilicon is deposited on the heated filament rods and the horizontal bridge, as a result of which the diameter thereof grows with time.
After the rods have cooled down, the reactor bell jar is opened and the rods are removed by hand or with the aid of specific apparatus, called deinstallation aids, for further processing or for temporary storage.
Both the storage and the further processing, in particular comminution of the rods, classification and packing of broken pieces, are generally effected under special environmental conditions in climate-controlled rooms, which prevents contamination of the product.
Between the time of opening of the reactor and storage or further processing, the material deposited, however, is exposed to environmental influences, especially dust particles.
The morphology and microstructure of the growing rod are determined by the parameters of the deposition process. The morphology of the deposited rods may vary from compact and smooth (as described, for example, in U.S. Pat. No. 6,350,313 B2) to very porous and fissured material (as described, for example, in US2010/219380 A1).
In the production of thick polycrystalline silicon rods (diameter>100 mm) in the Siemens reactors according the prior art, a relatively frequent observation is that the rods have regions with a very rough surface (“popcorn”). These rough regions have to be separated from the rest of the material, typically by visual checking after crushing, and are sold at much lower prices than the rest of the silicon rod.
Increasing the base parameters in the course of deposition (temperature of the rods, specific flow rate, concentration) generally leads to an increase in the deposition rate and hence to an improvement in the economic viability of the deposition process.
However, each of these parameters is subject to natural limits, exceedance of which disrupts the production process (according to the configuration of the reactor used, the limits are somewhat different).
If, for example, the chosen concentration of the silicon-containing component is too high, there may be homogeneous gas phase deposition.
The result of an excessively high rod temperature may be that the morphology of the silicon rods to be deposited is not compact enough to provide a sufficient cross-sectional area for the current flow as it rises with growing rod diameter. Excessively high current density can cause the melting of silicon.
In the case of rods of high diameter (120 mm and above), the choice of temperature is even more critical, since silicon within the rod can become liquid (because of the high temperature differences between the surface and the rod center), even when the morphology is compact.
Demands on the product from customers in the semiconductor and solar industries also distinctly restrict the ranges for the process parameters.
For example, FZ applications require silicon rods which are very substantially free of cracks, pores, discontinuities, fissures, etc. and hence are homogeneous, dense and solid. Moreover, for a better yield in FZ pulling, they should preferably have a particular microstructure. A material of this kind and the process for production thereof are described, for example, in US2008/286550 A1.
For the production of recharge rods and what are called cut rods, which are used principally in the CZ process to increase the crucible fill level, crack-free and low-stress raw polycrystalline silicon rods are likewise required.
For most applications, however, polycrystalline silicon rods are crushed to small pieces which are typically then classified by size. A process and an apparatus for comminution and sorting of polysilicon is described, for example, in US 2007/235574 A1.
US 2009081108 A1 discloses a workbench for manual sorting of polycrystalline silicon by size and quality. This involves implementation of an ionization system in order to neutralize electrostatic charges resulting from active air ionization. Ionizers permeate the cleanroom air with ions such that static charges at insulators and ungrounded conductors are dissipated.
US 2007235574 A1 discloses a device for comminuting and sorting polycrystalline silicon, comprising a feed for a coarse polysilicon fraction into a crushing system, the crushing system and a sorting system for classifying the polysilicon fraction, wherein the device is provided with a controller which allows variable adjustment of at least one crushing parameter in the crushing system and/or at least one sorting parameter in the sorting system. A polysilicon rod is placed on the crushing table of the pre-comminuter. Visual quality control of the rod for foreign bodies, deposits and morphology of the surface is carried out on the crushing table. The rod is placed on a crushing carriage, which conveys the rod automatically into the crushing chamber.
In the processing to chunks, rods with cracks and other material defects are accepted as starting material. However, the morphology of polycrystalline rods and chunks formed therefrom has a strong influence on the performance of the product. Typically, a porous and fissured morphology has an adverse effect on the crystallization characteristics.
This particularly affects the demanding CZ process, in which porous and fissured chunks were not usable because of the economically unacceptable yields.
Other crystallization processes (for example block casting, which is the most frequently used method for production of solar cells) are less morphology-sensitive. Here, the adverse effect of the porous and fissured material can be compensated for in economic terms by its lower production costs.
It is a problem that, in the production of compact materials, porous fractions sometimes also arise in the region of the top ends of the rods. In the case of demanding customer applications, however, porous rod fractions are unwanted, and so the reactor running curves have to be planned more “conservatively” than actually necessary, in order to avoid the last porous fractions as well.
On the other hand, the production of porous silicon also gives rise to compact fractions in the lower parts of the rods and on the rod edges facing the reactor wall.
In some cases, particular parts of rods are more heavily contaminated with impurities than others. EP 2479142 A1 discloses a process for producing a polycrystalline silicon chunk, comprising deposition of polycrystalline silicon on a support body in a reactor, withdrawing the polycrystalline silicon rod from the reactor and comminuting the silicon rod into silicon chunks, with removal of at least 70 mm from the electrode end of the polycrystalline silicon rod prior to the comminution. Here, part of the rod is thus removed before the comminution of the rod into chunks. The chunks obtained by comminution of the residual rod have a low content of chromium, iron, nickel, copper and cobalt.
These problems gave rise to the objectives of the invention.